Non-dispersive infrared gas analyzers typically utilize an infrared source to produce and direct infrared energy through an unknown gas mixture contained in a sample cell. The energy passing through the sample cell at certain predetermined wavelengths is detected and electrical signals are produced representative thereof. The predetermined wavelengths are selected to correspond with the characteristic frequency absorption of a gas or gasses of interest. The electrical signals are processed to produce an output indicating the concentration of one or more of the gases in the gas mixture in the sample cell.
One such analyzer is described in Passaro et al., U.S. Pat. No. 4,346,296. In this disclosure, an infrared source emits infrared radiation at relatively constant intensity over a relatively broad spectrum. The infrared radiation from the source is interrupted periodically by a chopper wheel. After passing through the sample cell, the chopped infrared radiation is detected by respective detectors. In each case the radiation is filtered by a narrow passband filter so that each detector is effectively sensitive only to the radiation of a particular narrow band of frequencies corresponding to a respective absorption frequency characteristic of the respective gas.
The respective detection signals are thus systematically related to the relative concentration of the respective gases. Because of the chopper wheel, these signals are AC signals at the chopper wheel frequency. The signals are then amplified, detected and filtered to produce corresponding DC signals.
Each filtered signal is applied to one input of a so-called span amplifier. A span amplifier typically comprises a summing amplifier which receives an offset reference input signal and a controllable feedback input signal. The offset reference signal offsets a zero point of the analyzer, and the feedback signal controls the span or the magnitude of the input signal required for providing a full-scale output.
The feedback signal is controlled by the span amplifier to balance out the offset signal to allow for the zero point to be determined. What is meant by "zero point" in this application is that the span amplifier in the absence of absorption of the incident infrared radiation should produce a zero output. As described in the aforesaid U.S. Pat. No. 4,346,296, the zero point of the span amplifier is adjusted by introducing a so-called zero gas in to the sample cell and adjusting the gain control on the feedback signal input to provide a zero meter reading. The zero gas is a gas, such as nitrogen, which is substantially non-absorptive of infrared energy, at least at the frequencies passed by the respective filters.
When a predetermined calibrating gas is introduced into the sample cell, the gain of the feedback signal is adjusted to some predetermined calibrated value. Then, when the gas to be analyzed is introduced, the output meter properly records or indicates the relative concentration of the respective constituent gases.
U.S. Pat. No. 4,687,934, entitled "Infrared Gas Analyzer With Automatic Zero Adjustment" in the names of Robert E. Passaro, Raymond E. Rogers and J. Craig Griffith, describes an automatic zero apparatus for an infrared gas analyzer. This apparatus comprises a comparator with a gain control which automatically controls the signal level of the detector signal from the span amplifier to reduce the output signal substantially to zero when the non-absorbent gas is within the sample cell. The aforementioned patent utilized a programmable microprocessor which provides a zeroing operation which is repeated after a predetermined period of time or upon a temperature drift of the gas analyzer above a predetermined level. The above-mentioned elements eliminate the necessity of having an operator adjust the system manually to a proper zero condition.
The above-mentioned patents disclose infrared gas analyzers that automatically provide a zero point over a predetermined period of time and upon a temperature drift above a predetermined level. It is also known, however, that gas analyzers of the above-mentioned type also exhibit set point instability due to variations in the temperature of the source, as well as variations in the temperature of the detectors. U.S. Pat. No. 4,398,091, issued in the name of Passaro, teaches a gas analyzer which compensates for these variations. Accordingly, oftentimes heaters are placed in the appropriate places within the analyzers and are then monitored to maintain the temperature of the gas analyzer at a fixed temperature to eliminate many of the drift corrections that are necessary when the ambient temperature changes. In so doing, the gas analyzer is more stable and therefore can more accurately measure the concentration levels of the various gases.
Generally, the monitoring of these elements is performed by a control circuit in conjunction with the processor providing a "set point" for the gas analyzer. What is meant by the set point in the context of the present application is the calibration point to which the device is set at a known gas and gas concentration, from which the gas measurements can be taken. To ensure that the readings of the gas analyzer are accurate, certain parameters such as temperature of the source and temperature of the heaters should be stable before the gas is measured. Accordingly, these parameters are monitored and adjusted to provide for the "set point" of the gas analyzer.
To control the set point, the measured temperature signals of the gas analyzer are provided to a set point circuit. These signals are compared to a reference signal. If the temperature signals do not correspond to the reference signal, then a processor within the gas analyzer will adjust the heaters' temperature until the appropriate output signal is obtained. Consequently, it is important that the set point be stable to ensure that the subsequent measurements made by the gas analyzer are accurate.
Typically, a differential amplifier circuit has been utilized to provide the stable set point in a gas analyzer by comparing the output signal of the temperature sensors associated with the heaters to some reference signal. If a predetermined signal is not present on the output of the amplifier, the processor then adjusts the temperature of the heater until the proper output voltage is present.
Although a differential amplifier works effectively as a set point control circuit within a gas analyzer, it requires a plurality of precision resistors for proper operation. Precision resistors are required because their values interact to provide the output voltage of the set point control circuit. As is well known, when a common input signal is provided to the two inputs of a differential amplifier, the differential mode output signal is zero. As a practical matter, however, it is also known that if both input terminals are at exactly the same potential, but the potential of both is varied together, some output voltage variation will occur. This output variation is called a common mode error signal. Accordingly, a common mode error signal represents an inaccuracy if the input signals are the same. This error can be minimized by ensuring that the input resistances are carefully matched. Therefore, precision resistors are utilized to minimize the common mode error.
It is known that precision resistors are expensive and can significantly increase the cost of a device. More particularly, it is very important in a gas analyzer to keep the overall costs as low as possible. It has been found that the use of a differential amplifier circuit in this context increases the need for the precision resistors and therefore undesirably increases the cost of the gas analyzer used therewith.
Accordingly, any system, device or apparatus that decreases the cost of the gas analyzer represents a significant advance over the art. More particularly, any arrangement which would limit the number of precision resistors necessary in a set point control circuit utilized in a gas analyzer and still maintain the stability thereof would represent a significant improvement over previously known circuits.
Broadly, it is an object of the present invention to provide an infrared gas analyzer with an improved set point control circuit.
It is a further object of the present invention to provide a circuit for providing a stable output signal for a given input signal over a predetermined temperature range and time interval.
It is also an object of the present invention to provide a gas analyzer that has a stable set point control circuit that is less expensive than previously known set point control circuitry utilized therewith.